Page 1

7 | Hemodynamic Disorders

7

Hemodynamic Disorders Bruce M. McManus • Michael F. Allard • Robert Yanagawa

Normal Circulation The Heart as a Two-Sided Pump Aorta and Arteries Microcirculation Endothelium Veins and Venules The Interstitium Lymphatics Disorders of Perfusion Hyperemia Hemorrhage Thrombosis Thrombosis in the Arterial System Thrombosis in the Heart Thrombosis in the Venous System Embolism Pulmonary Arterial Embolism Systemic Arterial Embolism

Infarction Infarction in Specific Locations Edema Congestive Heart Failure Pulmonary Edema Edema in Cirrhosis Nephrotic Syndrome Cerebral Edema Fluid Accumulation in Body Cavities Fluid Loss and Overload Dehydration Overhydration Shock Systemic Inflammatory Response Syndrome Genetic Polymorphisms Multiple Organ Dysfunction Syndrome

Normal Circulation

cally important parameters are cardiac output, perfusion pressure and peripheral vascular resistance.

Normal function and metabolism of organs and cells depend on an intact circulatory system for continuous delivery of oxygen, nutrients, hormones, electrolytes and water, as well as for removal of metabolic waste and carbon dioxide. The circulatory system is a vascular conduit made of a muscular pump connected to tubes (or blood vessels) that deliver blood to organs and tissues and return it to the heart to complete the circuit. Delivery and elimination at the cellular level are controlled by exchanges between the intravascular space, interstitial space, cellular space and lymphatic space, which occur via the smallest-diameter blood vessels in the body (the microcirculation).

Cardiac output is the volume of blood pumped by each ventricle per minute and represents the total blood flow in pulmonary and systemic circuits. Cardiac output is the product of heart rate and stroke volume and, as the cardiac index, is often adjusted for body surface area (in square meters) as an indicator of ventricular function. Perfusion pressure (also called driving pressure) is the difference in dynamic pressure between two points along a blood vessel. Blood flow to any segment of the circulation ultimately depends on arterial driving pressure. However, each organ can autoregulate flow and so determine the amount of blood it receives from the circulation. Such local control of perfusion depends on continuous modulation of microvascular beds by hormonal, neural, metabolic and hemodynamic factors. Peripheral vascular resistance is the sum of the factors that determine regional blood flow in each organ. Two thirds of the resistance in the systemic vasculature is determined by the arterioles.

The Heart Is a Two-Sided Pump With Vascular Circuits in Series

In this series circuit, the amount of blood handled by the right ventricle, which pumps blood to the lungs (pulmonary circulation), must, over time, exactly equal the amount of blood going through the left ventricle, which distributes blood to the body (systemic circulation). The hemodynami-

The sum of all regional flows equals the venous return, which in turn determines the cardiac output. Assessment of

267


268

RUBIN’S PATHOLOGY

the heart’s response to inflow (preload) and outflow (afterload) relies on cardiac reflexes as well as cardiac muscle integrity and neurohormonal regulation.

They also convert pulsatile flow into sustained regular flow. The latter function derives from the elastic properties of the aorta and the resistance produced by the arteriolar sphincters.

a link between oxygen demand and blood flow. In the heart, blood flow is adjusted on a second-to-second basis. Factors that mediate and link metabolic vasodilation to cellular metabolism include adenosine, other nucleotides, nitric oxide, certain prostaglandins, carbon dioxide and pH. The microcirculation is an important contributor to all forms of hyperemia and edema, and is a target in septic shock (see below). Vasoregulation in conducting arteries, resistance arteries and veins relies on delicate interactions between blood, endothelium, smooth muscle cells and surrounding stroma.

The Microcirculation Includes Arterioles, Capillaries and Venules

The Normal Endothelium Provides a Continuous Partition Between Blood and Tissues

The blood vessels of the microcirculation are less than 100 !M in diameter. Blood from an arteriole enters capillaries, which freely anastomose with each other (Fig. 7-1), either directly or through metarterioles. Capillary length, measured from terminal arteriole to collecting venule, ranges from 0.1 to 3 mm, averaging 1 mm. However, the path length by which blood cells traverse capillaries may actually be longer because of their extensive anastomoses. This fact is probably an important determinant of microvascular exchange of substances such as oxygen because it increases the time available for such exchange to occur. The large aggregate surface area of capillaries means that blood flow is slow, which further enhances microvascular exchange. Capillary density in a tissue also influences microvascular exchange by affecting diffusion distance. For example, in tissues with high oxygen demands, such as the heart, capillary density is very high. Entry into the capillary system is guarded by precapillary sphincters, except for thoroughfare channels, which bypass capillaries and are always open. Since not all capillaries are always open, blood flow to a structure can be increased by recruiting additional capillaries. The sum of blood flow through the capillary bed, the thoroughfare channels and the arteriovenous anastomoses determines the regional blood flow. The exact means by which an organ regulates blood flow according to its metabolic needs are still debated, but there is

Endothelial cells play important roles in anticoagulation, facilitation of migration of substances from blood to tissue and back, regulation of vessel tone (particularly that of resistance arteries) and regulation of vasopermeability (also see Chapters 2 and 10).

The Aorta and Arteries Transport Blood to the Organs

Veins and Venules Return Blood to the Heart Blood from the capillaries enters venules and eventually veins on its route back to the heart. Veins not only serve as a conduit for blood, but also act as a blood reservoir. Roughly 65% of the total blood volume resides in the venous system.

Interstitium Represents 15% of Total Body Volume The fluid between cells (interstitial fluid) helps to deliver nutrients to cells and eliminate cellular wastes. Most interstitial water is bound to a dense network of glycosaminoglycans.

Lymphatics Aid in the Reabsorption of Interstitial Fluid Most interstitial fluid reenters the circulation at the venous end of capillaries. A small portion is drained by lymphatics. Lymphatic capillaries conduct lymph from the periphery to the central venous system via the thoracic duct. Normal oscillatory constrictions and relaxations of lymphatic vessels contribute to steady return of lymph fluid to the central circulation. Lymph is important for transport of molecules too large to return to the circulation through blood capillaries.

Disorders of Perfusion Hemodynamic disorders are characterized by disturbed perfusion that may result in organ and cellular injury.

Hyperemia Is an Excess of Blood in an Organ Hyperemia may be caused either by an increased supply of blood from the arterial system (active hyperemia) or by impaired exit of blood through venous pathways (passive hyperemia or congestion).

FIGURE 7-1. Microcirculation. Photomicrograph of myocardium showing capillaries and venules (arrow).

Active Hyperemia Active hyperemia is augmented supply of blood to an organ. It is usually a physiologic response to increased functional


demand, as in the heart and skeletal muscle during exercise. Neurogenic and hormonal influences play a role in active hyperemia (e.g., the blushing bride and the menopausal flush). Although the utility of vasodilation is not always clear, cutaneous hyperemia in febrile states serves to dissipate heat. In addition, skeletal muscle may increase its blood flow (and thus oxygen delivery) 20-fold during exercise. The increased blood supply occurs by arteriolar dilation and recruitment of unperfused capillaries. The most striking active hyperemia occurs in association with inflammation. Vasoactive materials released by inflammatory cells (see Chapter 2) cause blood vessels to dilate; in the skin this contributes to the classic “tumor, rubor and calor” of inflammation. In pneumonia, for example, alveolar capillaries are engorged with erythrocytes as a hyperemic response to inflammation. Because inflammation can also damage endothelial cells and increase capillary permeability, such hyperemia is often accompanied by edema and local extravasation of erythrocytes. Reactive hyperemia occurs after temporary interruption of blood supply (ischemia). Removal of the obstruction is followed by active hyperemia, probably due to ischemic tissue injury and release of inflammatory agents such as histamine. The degree and duration of hyperemia is proportional to the period of occlusion until a plateau of hyperemic response is reached.

FIGURE 7-2. Passive congestion of lung. Hemosiderin-laden macrophages in the lung of a patient with congestive heart failure.

Passive Hyperemia (Congestion) Passive hyperemia, or congestion, is engorgement of an organ with venous blood. Acute passive congestion is clinically a consequence of acute left or right ventricular failure. Regarding the former, resultant pulmonary venous engorgement leads to pulmonary edema, or accumulation of a transudate within the alveolar space. With acute failure of the right ventricle, the liver can become severely congested. A generalized increase in venous pressure, typically from chronic heart failure, results in slower blood flow and a consequent increase in blood volume in many organs, including liver, spleen and kidneys. In the past, heart failure from rheumatic mitral stenosis was a common cause of generalized venous congestion, but with the decline in the prevalence of rheumatic fever and the advent of surgical valve replacement, such cases are unusual. Congestive heart failure secondary to coronary artery disease and hypertension and right-sided failure due to pulmonary disease are now more common. Passive congestion may also be confined to a limb or an organ as a result of more-localized obstruction to venous drainage. Examples include deep thrombosis in the leg veins, resulting in edema of the lower extremity and thrombosis of hepatic veins (Budd-Chiari syndrome) with secondary chronic passive congestion of the liver (see Chapter 14).

269

Fluid is forced from the blood into the alveolar airspaces. The resulting pulmonary edema (Fig. 7-3) interferes with gas exchange in the lung. Fibrosis increases in the interstitium of the lung. The presence of fibrosis and iron is viewed grossly as a firm, brown lung (brown induration). Pulmonary hypertension occurs when the back-pressure from the pulmonary venous circuit is transmitted to the pulmonary arterial system. This may lead to right-sided heart failure and consequent generalized systemic venous congestion.

The morphologic changes associated with chronic passive congestion of the lungs are discussed in Chapter 12. LIVER: The hepatic veins empty into the vena cava immediately inferior to the heart, so the liver is particularly vulnerable to acute or chronic passive congestion (see Chapter 14). Increased venous pressure causes the central veins of hepatic lobules to dilate and is transmitted to hepatic sinusoids, which dilate, causing centrilobular hepatocytes to undergo pressure atrophy (Fig. 7-4A). Grossly, the cut surface of a chronically congested liver has dark foci of

LUNGS: Chronic left ventricular failure impedes blood flow out of the lungs and leads to chronic passive pulmonary congestion. As a result, pressure in alveolar capillaries increases and these vessels become engorged with blood. Increased alveolar capillary pressure has four major consequences: ■

Microhemorrhages release erythrocytes into alveolar spaces, where they are phagocytosed and degraded by alveolar macrophages. The released iron, in the form of hemosiderin, remains in these macrophages, which are then called “heart failure cells” (Fig. 7-2).

FIGURE 7-3. Pulmonary edema. A patient with congestive heart failure shows pink-staining fluid in the alveoli.

7 | Hemodynamic Disorders

CHAPTER 7: HEMODYNAMIC DISORDERS | DISORDERS OF PERFUSION


270

RUBIN’S PATHOLOGY

B

A

FIGURE 7-4. Passive congestion of liver. A. A photomicrograph of liver shows dilated centrilobular sinusoids. The intervening plates of hepatocytes exhibit pressure atrophy. B. A gross photograph of liver shows nutmeg appearance, reflecting congestive failure of the right ventricle. C. Late changes in chronic passive congestion characterized by dilated sinusoids (arrows) and fibrosis (note the blue staining of collagen in this trichrome stain). Proliferated bile ducts are on the right.

centrilobular congestion surrounded by paler zones of unaffected peripheral portions of the lobules. The result is a reticulated appearance that resembles a cross-section of a nutmeg (“nutmeg liver”) (Fig. 7-4B). In severe cases associated with acute right ventricular failure, frank hemorrhagic necrosis of hepatocytes in centrilobular zones is conspicuous. Prolonged hepatic venous congestion eventually leads to thickening of central veins and centrilobular fibrosis. Only in the most extreme cases of venous congestion (e.g., constrictive pericarditis or tricuspid stenosis) is the fibrosis sufficiently generalized and severe to be termed cardiac cirrhosis (Fig. 7-4C). SPLEEN: Increased intravascular pressure in the liver, whether from cardiac failure or an intrahepatic obstruction to blood flow (e.g., cirrhosis), leads to higher pressure in the splenic vein and congestion of the spleen. The organ becomes enlarged and tense, and the cut section oozes dark blood. In long-standing congestion, diffuse splenic fibrosis develops, as do iron-containing, fibrotic and calcified foci of old hemorrhage (Gamna-Gandy bodies). Such a spleen may weigh 250 to 750 g, compared with a normal weight of 150 g. The enlarged spleen sometimes shows excessive functional activity—termed hypersplenism—which leads to hematologic abnormalities (e.g., thrombocytopenia). EDEMA AND ASCITES: Venous congestion impedes capillary blood flow, thereby increasing hydrostatic pressure and promoting edema formation (see below for a discussion of mech-

C

anisms of edema formation). Accumulation of edema fluid in heart failure is particularly noticeable in dependent tissues— legs and feet in ambulatory patients and the back in bedridden persons. Ascites is accumulation of fluid in the peritoneal space and reflects (among other factors) lack of tissue rigor, a condition in which there is no countervailing external pressure to oppose hydrostatic pressure within the blood vessels.

Hemorrhage Is a Discharge of Blood out of the Vascular Compartment Blood can be released from the circulation to the exterior of the body or into nonvascular body spaces. The most common and obvious cause is trauma. Severe atherosclerosis may so weaken the wall of the abdominal aorta that it balloons to form an aneurysm, which then may rupture and bleed into the retroperitoneal space (see Chapter 10). In the same way, an aneurysm may complicate a congenitally weak cerebral artery (berry aneurysm) and lead to subarachnoid hemorrhage (see Chapter 28). Certain infections (e.g., pulmonary tuberculosis) and invasive neoplasms may erode blood vessels and lead to hemorrhage. Hemorrhage also results from damage to capillaries. For instance, rupture of capillaries by blunt trauma leads to a bruise. Increased venous pressure also causes extravasation of blood from pulmonary capillaries. Vitamin C deficiency is associated with capillary fragility and bleeding, due to a defect in the supporting connective tissue structures. The


CHAPTER 7: HEMODYNAMIC DISORDERS | THROMBOSIS

271

■ ■ ■ ■ ■ ■

Hematoma: Hemorrhage into soft tissue. Such collections of blood can be merely painful, as in a muscle bruise, or fatal, if located in the brain. Hemothorax: Hemorrhage into the pleural cavity. Hemopericardium: Hemorrhage into the pericardial space. Hemoperitoneum: Bleeding into the peritoneal cavity. Hemarthrosis: Bleeding into a joint space. Purpura: Diffuse superficial hemorrhages in the skin, up to 1 cm in diameter. Ecchymosis: A large superficial hemorrhage in the skin (“black and blue” mark; Fig. 7-5). The initially purple discoloration turns green and then yellow before resolving. This sequence of events reflects progressive oxidation of bilirubin released from the hemoglobin of degraded erythrocytes. A good example of an ecchymosis is a “black eye.” Petechiae: Pinpoint hemorrhages, usually seen in the skin or conjunctiva (Fig. 7-6). This lesion reflects rupture of a capillary or arteriole and occurs in conjunction with coagulopathies or vasculitis. Petechiae may also be produced by microemboli from infected heart valves (bacterial endocarditis).

7 | Hemodynamic Disorders

capillary barrier by itself does not suffice to contain blood within the intravascular space. The minor trauma imposed on small vessels and capillaries by normal movement requires an intact coagulation system to prevent hemorrhage. Thus, a severe decrease in the number of platelets (thrombocytopenia) or deficiency of a coagulation factor (e.g., factor VIII in hemophilia A or von Willebrand factor in von Willebrand disease) is associated with spontaneous hemorrhage without apparent trauma (see Chapters 10 and 20). A person may exsanguinate into an internal cavity, as in gastrointestinal hemorrhage from a peptic ulcer (arterial hemorrhage) or esophageal varices (venous hemorrhage). In such cases, large amounts of fresh blood fill the entire gastrointestinal tract. Bleeding into a serous cavity can result in accumulation of a large amount of blood, even to the point of exsanguination. A few definitions are in order: FIGURE 7-6. Petechiae. Periorbital microhemorrhages (arrows) appear as punctate red foci.

Thrombosis A thrombus is an aggregate of coagulated blood containing platelets, fibrin and entrapped cellular elements, within a vascular lumen. Formation of a thrombus is thrombosis. A thrombus by definition adheres to vascular endothelium and should be distinguished from a simple blood clot, which reflects only activation of the coagulation cascade and can form in vitro or even postmortem. Similarly, a thrombus differs from a hematoma, which results from hemorrhage and subsequent clotting outside the vascular system. Thrombus formation and the coagulation cascade are discussed in more detail in Chapters 10 and 20. The pathogenesis of venous and arterial thrombosis has classically been considered distinct, but recent epidemiologic evidence demonstrates commonalities in risk factors. Here we present the causes and consequences of thrombosis in these different vascular sites.

Thrombosis in the Arterial System Is Usually Due to Atherosclerosis ETIOLOGIC FACTORS: The vessels most commonly

involved in arterial thrombosis are coronary, cerebral, mesenteric and renal arteries and arteries of the lower extremities. Less commonly, arterial thrombosis may occur in other disorders, including inflammation of arteries (arteritis), trauma and blood diseases. Thrombi are common in aneurysms (localized dilations of the lumen) of the aorta and its major branches, in which the distortion of blood flow, combined with intrinsic vascular disease, promotes thrombosis. The pathogenesis of arterial thrombosis involves principally three factors: ■

FIGURE 7-5. Ecchymosis. Superficial diffuse hemorrhage (arrows) on the thigh caused by blunt force trauma.

Damage to endothelium, usually by atherosclerosis, disturbs the anticoagulant properties of the vessel wall and serves as a nidus for platelet aggregation and fibrin formation. Alterations in blood flow, whether from turbulence in an aneurysm or at sites of arterial bifurcation, is conducive to thrombosis. Slowing of blood flow in narrowed arteries favors thrombosis.


272 ■

RUBIN’S PATHOLOGY

Increased coagulability of the blood, as seen in polycythemia vera or in association with some cancers, leads to an increased risk of thrombosis.

A major risk factor for venous thrombosis is immobilization after surgery or after leg casting. Other risk factors include metabolic syndrome, which includes obesity, hyperglycemia, insulin resistance, dyslipidemia and hypertension (see Chapter 22); advanced age; tobacco use; previous thrombosis; and cancer.

MOLECULAR PATHOGENESIS: Genetic stud-

ies of arterial thrombosis have not identified gene mutations that confer risk upon large populations, unlike venous thrombosis, for which mutations in several genes have been identified (see below). This is likely due to the complex polygenetic and multifactorial nature of atherosclerotic plaque formation, erosion or rupture. Weak associations have been shown between certain coagulation factors, fibrinolytic and inflammatory mediators and arterial thrombosis (e.g., factor VII and fibrinogen). Hyperhomocysteinemia is associated with atherosclerotic coronary artery disease and cardiac ischemia. In regards to genetics associated with arterial thrombosis, identification of affected family members may guide screening and treatment.

FIGURE 7-8. Canalization of thrombus. Photomicrograph of the left anterior descending coronary artery shows severe atherosclerosis and canalization.

■ ■

PATHOLOGY: An arterial thrombus attached to a

vessel wall is initially soft and friable, with fine alternating red bands composed principally of erythrocytes, and yellowish bands composed of platelets and fibrin (the lines of Zahn) (Fig. 7-7). Once formed, arterial thrombi have several possible outcomes. ■ ■ ■

Lysis, owing to the potent thrombolytic activity of the blood Propagation (i.e., an increase in its size), because a thrombus serves as a focus for further thrombosis Organization, the eventual invasion of connective tissue elements, which causes a thrombus to become firm and grayish white

Canalization, by which new lumina lined by endothelial cells form in an organized thrombus (Fig. 7-8) Embolization, when part or all of the thrombus becomes dislodged, travels through the circulation and lodges in a blood vessel at a distance from the site of thrombus formation (see below for further discussion)

The organized structure of thrombi reflects a tight interaction between platelets and fibrin and differs in appearance from a postmortem clot or one formed in a test tube. Determination of whether a clot formed during life (antemortem clot) or after death (postmortem clot) is often important in a medical autopsy and in forensic pathology. Lines of Zahn stabilize a thrombus formed during life, while a postmortem clot has a more gelatinous structure. Postmortem clots occur in stagnant blood in which gravity fractionates the blood. The part of the clot containing many red blood cells has a reddish, gelatinous appearance, and is referred to as “currant jelly.” The overlying clot is firmer and yellow-white, representing coagulated plasma without red blood cells. It is called “chicken fat” because of its color and consistency. CLINICAL FEATURES: Arterial thrombosis due to

atherosclerosis is the most common cause of death in Western industrialized countries. Since most arterial thrombi occlude the vessel, they often lead to ischemic necrosis of tissue supplied by that artery (i.e., an infarct). Thus, thrombosis of a coronary or cerebral artery results in myocardial infarct (heart attack) or cerebral infarct (stroke), respectively. Other end-arteries that are affected often by atherosclerosis and thrombosis include the mesenteric arteries (intestinal infarction), renal arteries (kidney infarcts) and arteries of the leg (gangrene).

FIGURE 7-7. Arterial thrombus. Gross photograph of a thrombus from an aortic aneurysm shows the laminations of fibrin and platelets known as the lines of Zahn.

Thrombosis in the Heart Develops on the Endocardium As in the arterial system, endocardial injury and changes in blood flow in the heart may lead to mural thrombosis (i.e., a


CHAPTER 7: HEMODYNAMIC DISORDERS | THROMBOSIS

273

■ ■ ■ ■

Stasis (heart failure, chronic venous insufficiency, postoperative immobilization, prolonged bed rest, hospitalization and travel, particularly in prolonged airplane trips) Injury and inflammation (trauma, surgery, childbirth, infection) Hypercoagulability (oral contraceptives, late pregnancy, cancer, inherited thrombophilic disorders [see Chapter 20]) Advanced age (venous varicosities, phlebosclerosis) Sickle cell disease (see Chapter 20) MOLECULAR PATHOGENESIS: Genetic factors

account for approximately 60% of the risk for deep venous thrombosis (DVT) according to twin and family studies, but, to date, few genetic associations have been identified. In the United States, blacks are more susceptible to development of DVT as compared to whites, whereas Asians and Hispanics are less so. The most common gene variant associated with venous thrombosis is factor V Leiden mutation, which results in a poor inactivation and anticoagulant response to activated protein C. It is present in 3% of the general population but accounts for 20% to 40% of cases of venous thrombosis, making it the most common genetic cause of venous thrombosis. It increases the risk for venous thrombosis sevenfold in heterozygotes and 80-fold in homozygotes. Prothrombotic factors further increase the risk, including oral contraceptive use, pregnancy, estrogen therapy, diabetes mellitus, malignancy, obesity and immobilization. Another common but mild risk factor for venous thrombosis is the prothrombin G20210A mutation. Deficiencies in proteins C and S and antithrombin are strong risk factors for DVT, but only in 5% to 10% of cases. Patient genetic profiles may help with risk stratification to identify high-risk prothrombotic groups for postoperative DVT to target use of Doppler ultrasound or prophylactic anticoagulant therapy.

FIGURE 7-9. Endocarditis. The anterior leaflet of the mitral valve is damaged by a friable bacterial vegetation.

thrombus adhering to the underlying wall of the heart). Disorders in which mural thrombi occur include: ■

Myocardial infarction: Adherent mural thrombi form in the left ventricular cavity over areas of myocardial infarction, owing to damaged endocardium and alterations in blood flow associated with a poorly functional or adynamic segment of the myocardium. Atrial fibrillation: Disordered atrial rhythm (atrial fibrillation) slows blood flow and impairs left atrial contractility, which predisposes to formation of mural thrombi in atria. Cardiomyopathy: Primary myocardial diseases are associated with mural thrombi in the left ventricle, presumably because of endocardial injury and altered hemodynamics associated with poor myocardial contractility. Endocarditis: Small thrombi, vegetations, may also develop on cardiac valves, usually mitral or aortic, that are damaged by a bacterial infection (bacterial endocarditis) (Fig. 7-9). Occasionally, vegetations form in the absence of valve infection, on a mitral or tricuspid valve injured by systemic lupus erythematosus (Libman-Sacks endocarditis). In chronic wasting states, as in terminal cancer, large, friable vegetations may appear on cardiac valves (marantic endocarditis), possibly reflecting a hypercoagulable state. The major complication of thrombi in any location in the heart is detachment of fragments and their lodging in blood vessels at distant sites (embolization).

PATHOLOGY: Most (!90%) venous thromboses occur in deep veins of the legs; the rest usually involve pelvic veins. Most venous thrombi begin in the calf veins, frequently in the sinuses above the venous valves. There, venous thrombi have several potential fates: ■ ■

Thrombosis in the Venous System Is Multifactorial The term currently used to designate the formation of blood clots in the venous system is deep venous thrombosis, which replaces older terminology (phlebothrombosis or, in the absence of inflammation, thrombophlebitis). The most common manifestation of the disorder, namely, thrombosis of the deep venous system of the legs, is aptly described by the current appellation. ETIOLOGIC FACTORS: Deep venous thrombosis is

caused by the same factors that favor arterial and cardiac thrombosis—endothelial injury, stasis and a

Lysis: They may remain small and are eventually lysed, posing no further threat to health. Organization: Many undergo organization similar to those of arterial origin. Small, organized venous thrombi may be incorporated into the vessel wall; larger ones may undergo canalization, with partial restoration of venous drainage. Propagation: Venous thrombi often serve as a nidus for further thrombosis and thereby propagate proximally to involve the larger iliofemoral veins (Fig. 7-10). Embolization: Large venous thrombi or those that have propagated proximally represent a significant hazard to life: they may dislodge and be carried to the lungs as pulmonary emboli. CLINICAL FEATURES: Small thrombi in the calf

veins are ordinarily asymptomatic, and even larger thrombi in the iliofemoral system may cause no

7 | Hemodynamic Disorders

hypercoagulable state. Conditions that favor the development of deep venous thrombosis include:


274

RUBIN’S PATHOLOGY

FIGURE 7-10. Venous thrombus. The femoral vein has been opened to reveal a large thrombus within the lumen.

symptoms. Some patients have calf tenderness, often associated with forced dorsiflexion of the foot (Homan sign). Occlusive thrombosis of femoral or iliac veins leads to severe congestion, edema and cyanosis of the lower extremity. Symptomatic deep venous thrombosis is treated with systemic anticoagulants, and thrombolytic therapy may be useful in selected cases. In some cases, a filter may be inserted into the vena cava to prevent pulmonary embolization. The function of venous valves is always impaired in a vein subjected to thrombosis and organization. As a result, chronic deep venous insufficiency (i.e., impaired venous drainage) is virtually inevitable. If a lesion is restricted to a small segment of the deep venous system, the condition may remain asymptomatic. However, more extensive involvement leads to pigmentation, edema and induration of leg skin. Ulceration above the medial malleolus can occur and is often difficult to treat. Venous thrombi elsewhere may also be dangerous. Thrombosis of mesenteric veins can cause hemorrhagic smallbowel infarction; thrombosis of cerebral veins may be fatal; hepatic vein thrombosis (Budd-Chiari syndrome) may destroy the liver. Inherited disorders of blood clotting may lead to susceptibility to these types of events. These diseases are covered in detail in Chapter 20.

between 50,000 and 100,000 deaths. It represents an important diagnostic and therapeutic challenge. In fact, pulmonary thromboemboli are reported in more than half of all autopsies. Furthermore, this complication occurs in 1% to 2% of postoperative patients over the age of 40. The risk after surgery increases with advancing age, obesity, length of operative procedure, postoperative infection, cancer and preexisting venous disease. Most pulmonary emboli (90%) arise from deep veins of the lower extremities; most fatal ones form in iliofemoral veins (Fig. 7-11). Only half of patients with pulmonary thromboembolism have signs of deep vein thrombosis. Some thromboemboli arise from the pelvic venous plexus and others from the right side of the heart. Emboli are also derived from thrombi around indwelling lines in the systemic venous system or pulmonary artery. The upper extremities are rarely sources of thromboemboli. The clinical features of pulmonary embolism are determined by the size of the embolus, the health of the patient and whether embolization occurs acutely or chronically. Acute pulmonary embolism is divided into the following syndromes:

Infected venous catheter Pulmonary embolus without infarction

Tumor emboli (e.g., renal cell carcinoma)

Thromboembolus of main pulmonary artery (saddle embolus),shock Pulmonary embolus with infarction Infection (air, foreign material)

Amniotic fluid embolism

Fracture with fat embolism Deep venous thrombosis

Embolism Embolism is passage through venous or arterial circulations of any material that can lodge in a blood vessel and obstruct its lumen. The most common embolus is a thromboembolus—a thrombus formed in one location that detaches from a vessel wall at its point of origin and travels to a distant site.

Pulmonary Arterial Embolism Is Common and Is Potentially Fatal Pulmonary embolism is estimated to be responsible for 300,000 hospitalizations in the United States yearly and

FIGURE 7-11. Sources and effects of venous emboli.


CHAPTER 7: HEMODYNAMIC DISORDERS | EMBOLISM ■ ■ ■ ■

275

Asymptomatic small pulmonary emboli Transient dyspnea and tachypnea without other symptoms Pulmonary infarction, with pleuritic chest pain, hemoptysis and pleural effusion Cardiovascular collapse with sudden death

7 | Hemodynamic Disorders

Chronic pulmonary embolism, with numerous (usually asymptomatic) emboli lodged in small arteries of the lung, can lead to pulmonary hypertension and right-sided heart failure (see below).

Massive Pulmonary Embolism One of the most dramatic calamities complicating hospitalization is the sudden collapse and death of a patient who had appeared to be well on the way to an uneventful recovery. The cause of this catastrophe is often massive pulmonary embolism due to release of a large deep venous thrombus from a lower extremity. Classically, a postoperative patient succumbs upon getting out of bed for the first time. The muscular activity dislodges a thrombus that formed as a result of the stasis from prolonged bed rest. Excluding deaths related to surgery itself, pulmonary embolism is the most common cause of death after major orthopedic surgery and is the most frequent nonobstetric cause of postpartum death. It also is an especially common cause of death in patients who suffer from chronic heart and lung diseases and in those subjected to prolonged immobilization for any reason. Prolonged immobilization associated with air travel can also lead to venous thrombosis and, occasionally, sudden death from a pulmonary embolus. A large pulmonary embolus may lodge at the bifurcation of the main pulmonary artery (saddle embolus), obstructing blood flow to both lungs (Fig. 7-12). Large lethal emboli may also be found in the right or left main pulmonary arteries or their first branches. Multiple smaller emboli may lodge in secondary branches and prove fatal. With acute obstruction of more than half of the pulmonary arterial tree, the patient often experiences immediate severe hypotension (or shock) and may die within minutes. The hemodynamic consequences of such massive pulmonary embolism are acute right ventricular failure from sudden obstruction of outflow and pronounced reduction in left ventricular cardiac output, secondary to the loss of right ventricular function. The low cardiac output is responsible for the sudden appearance of severe hypotension.

Pulmonary Infarction Small pulmonary emboli are not ordinarily lethal. They tend to lodge in peripheral pulmonary arteries. Sometimes (15% to 20% of all pulmonary emboli) they produce lung infarcts. Clinically, pulmonary infarction is usually seen in the context of congestive heart failure or chronic lung disease, because the normal dual circulation of the lung ordinarily protects against ischemic necrosis; since the bronchial artery supplies blood to the necrotic area, pulmonary infarcts are typically hemorrhagic. They tend to be pyramidal, with the base of the pyramid on the pleural surface. Patients experience cough, stabbing pleuritic pain, shortness of breath and occasional hemoptysis. Pleural effusion is common and often bloody. With time, the blood in the infarct is resorbed and the center of the infarct becomes pale. Granulation tissue forms at the edge of the infarct, after which it is organized to form a fibrous scar.

FIGURE 7-12. Pulmonary embolism. The main pulmonary artery and its bifurcation have been opened to reveal a large saddle embolus.

Pulmonary Embolism Without Infarction Since the lung is supplied by both the bronchial arteries and the pulmonary artery, most (75%) small pulmonary emboli do not produce infarcts. Although most small emboli do not attract clinical attention, a few lead to a syndrome characterized by dyspnea, cough, chest pain and hypotension. Rarely (3%), recurrent pulmonary emboli cause pulmonary hypertension by mechanical blockage of the arterial bed. In this circumstance, reflex vasoconstriction and bronchial constriction, owing to release of vasoactive substances, may contribute to a reduction in size of the functional pulmonary vascular bed. In the clinical syndrome of “partial infarction,” patients have the clinical and radiologic findings of pulmonary infarction due to thromboembolism. However, the lesion resolves instead of contracting to leave a scar. In such cases, hemorrhage and necrosis of the lung tissue in the affected area occur, but the tissue framework remains. Collateral circulation maintains tissue viability and enables its regeneration.

Fate of Pulmonary Thromboemboli Small pulmonary emboli may completely resolve, depending on (1) the embolic load, (2) the adequacy of the pulmonary vascular reserve, (3) the state of the bronchial collateral circulation and (4) the thrombolytic process. Alternatively, thromboemboli may become organized and leave strings of fibrous tissue attached to a vessel wall in the lumen of pulmonary arteries. Radiologic studies have indicated that half of all pulmonary thromboemboli are resorbed and organized within 8 weeks, with little narrowing of the vessels.


276

RUBIN’S PATHOLOGY

Paradoxical Embolism Paradoxical embolism refers to emboli that arise in the systemic venous circulation but bypass the lungs by traveling through an incompletely closed foramen ovale, subsequently entering the left side of the heart and blocking flow to the systemic arteries. Since left atrial pressure usually exceeds that in the right, most of these cases occur in the context of a rightto-left shunt (see Chapter 11).

Systemic Arterial Embolism Often Causes Infarcts Thromboembolism The heart is the most common source of arterial thromboemboli (Fig. 7-13), which usually arise from mural thrombi (Fig. 7-14) or diseased valves. These emboli tend to lodge at points where vessel lumens narrow abruptly (e.g., at bifurcations or near an atherosclerotic plaque). The viability of tissue supplied by the affected vessel depends on the availability of collateral circulation and on the fate of the embolus itself. The thromboembolus may propagate locally and lead to more severe obstruction, or it may fragment and lyse. Organs that suffer the most from arterial thromboembolism include:

FIGURE 7-14. Mural thrombus of the left ventricle. A laminated thrombus adheres to the endocardium overlying a healed aneurysmal myocardial infarct.

■ ■

■ ■

Brain: Arterial emboli to the brain cause ischemic necrosis (strokes). Intestine: In the mesenteric circulation, emboli cause bowel infarction, which manifests as an acute abdomen and requires immediate surgery. Lower extremity: Embolism to an artery of the leg leads to sudden pain, absence of pulses and a cold limb. In some cases, the limb must be amputated. Kidney: Renal artery embolism may infarct an entire kidney but more commonly causes small peripheral infarcts. Heart: Coronary artery embolism and resulting myocardial infarcts occur but are rare.

The more common sites of infarction from arterial emboli are summarized in Fig. 7-15.

Air Embolism

FIGURE 7-13. Sources of arterial emboli.

Air may enter the venous circulation through neck wounds, thoracocentesis or punctures of the great veins during invasive procedures or hemodialysis. Small amounts of circulating air in the form of bubbles are of little consequence, but quantities of 100 mL or more can lead to sudden death. Air bubbles tend to coalesce and physically obstruct blood flow in the right side of the heart, the pulmonary circulation and the brain. Histologically, air bubbles appear as empty spaces in capillaries and small vessels of the lung. People exposed to increased atmospheric pressure, such as scuba divers and workers in underwater occupations (e.g., tunnels, drilling platform construction), are subject to decompression sickness, a unique form of gas embolism. During descent, large amounts of inert gas (nitrogen or helium) dissolve in bodily fluids. When the diver ascends, this gas is released from solution and exhaled. However, if ascent is too rapid, gas bubbles form in the circulation and within tissues, obstructing blood flow and directly injuring cells. Air embolism is the second most common cause of death in sport diving (drowning is the first). Acute decompression sickness, “the bends,” is characterized by temporary muscular and joint pain owing to small vessel obstruction in these tissues. However, severe involvement of cerebral blood vessels may cause coma or even death.


CHAPTER 7: HEMODYNAMIC DISORDERS | EMBOLISM

277

high thromboplastin activity of amniotic fluid may initiate a potentially fatal consumptive coagulopathy.

amniotic fluid embolism can be dramatic, with sudden onset of cyanosis and shock, followed by coma and death. If the mother survives this acute episode, she may die of disseminated intravascular coagulation. Should she overcome this complication, she is at substantial risk of developing acute respiratory distress syndrome (see Chapter 12). Minor amniotic fluid embolism is probably common and asymptomatic, since autopsies of mothers who have died of other causes in the perinatal period frequently show evidence of this complication.

Fat Embolism Fat embolism is release of emboli of fatty marrow (Fig. 7-17) into damaged blood vessels after severe trauma to fatcontaining tissue, particularly accompanying bone fractures. In most instances, fat embolism is clinically inapparent. However, severe fat embolism leads to fat embolism syndrome 1 to 3 days after the injury. In its most severe form, which may be fatal, this syndrome is characterized by respiratory failure, mental changes, thrombocytopenia and widespread petechiae. Chest radiography reveals diffuse opacity of the lungs, which may progress to a “whiteout� typical of acute respiratory distress syndrome. At autopsy, innumerable fat globules are seen in the pulmonary microvasculature (Fig. 7-17B), the brain and sometimes other organs. The lungs typically exhibit the changes of acute respiratory distress syndrome (see Chapter 12). The lesions in the brain include cerebral edema, small hemorrhages and occasionally microinfarcts.

FIGURE 7-15. Common sites of infarction from arterial emboli.

Caisson disease refers to decompression sickness in which vascular obstruction causes multiple foci of ischemic (avascular) necrosis of bone, particularly affecting the head of the femur, tibia and humerus. This complication was originally described in construction workers in diving bells (or caissons).

Amniotic Fluid Embolism Amniotic fluid containing fetal cells and debris may enter the maternal circulation through open uterine and cervical veins. This rare maternal complication of childbirth usually occurs at the end of labor, and can be catastrophic when it occurs. The emboli are composed of the solid epithelial constituents (squames) contained in the amniotic fluid (Fig. 7-16). The

FIGURE 7-16. Amniotic fluid embolism. A section of lung shows a pulmonary artery filled with epithelial squames.

7 | Hemodynamic Disorders

CLINICAL FEATURES: The clinical presentation of


278

RUBIN’S PATHOLOGY

A

B FIGURE 7-17. Fat embolism. A. The lumen of a small pulmonary artery is occluded by a fragment of bone marrow consisting of fat cells and hematopoietic elements. B. A frozen section of lung stained with Sudan red shows capillaries occluded by red-staining fat emboli.

Fat embolism is usually considered a direct consequence of trauma, with fat entering ruptured capillaries at the site of the fracture. However, this explanation may be too simplistic. It has been suggested that hemorrhage into the marrow and perhaps also into the subcutaneous fat raises interstitial pressure above capillary pressure, forcing fat into the circulation. Moreover, there is more fat in the pulmonary vascular system than can be accounted for by simple transfer of fat from peripheral depots, and the chemical composition of the fat in the lung differs from that in tissue. Finally, there is a discrepancy between the frequency of fat embolism and bone marrow embolism.

Bone Marrow Embolism Bone marrow emboli to the lungs, complete with hematopoietic cells and fat, are often encountered at autopsy after cardiac resuscitation, during which procedure fractures of the sternum and ribs are common. These emboli also occasionally occur after fractures of long bones. In most cases no symptoms are attributed to bone marrow embolism.

Miscellaneous Pulmonary Emboli Intravenous drug abusers who use talc as a carrier for illicit drugs may introduce it into the lung via the bloodstream. Talc emboli produce granulomatous responses in the lungs (Fig. 7-18). Cotton emboli are surprisingly common and are due to cleansing of the skin prior to venipuncture. Schistosomiasis may be associated with embolization of ova to the lungs from bladder or gut, in which case they incite a foreign body granulomatous reaction. Tumor emboli are occasionally seen in the lung during hematogenous dissemination of cancer.

Infarction Infarction is the process by which coagulative necrosis develops in an area distal to occlusion of an end-artery. The necrotic zone is an infarct. Infarcts of vital organs such as heart, brain and intestine are serious medical conditions and are major causes of morbidity and mortality. If the victim survives, the infarct heals with a scar. Partial arterial occlusion (i.e., stenosis) occasionally causes necrosis, but it more commonly leads to atrophic changes associated with chronic ischemia (see Chapter 1). For example, in the heart these changes include vacuolization of cardiac myocytes, atrophy, loss of muscle cell myofibrils and interstitial fibrosis. PATHOLOGY: The gross and microscopic appear-

ance of an infarct depends on its location and age. Upon arterial occlusion, the area supplied by the vessel rapidly becomes swollen and deep red. Microscopically, vascular dilation and congestion and occasionally interstitial hemorrhage are noted. Subsequently, several types of infarcts are distinguishable by gross examination. Pale infarcts are typical in the heart, kidneys and spleen (Fig. 7-19), although certain renal infarcts may be cystic. Dry gangrene of the leg due to arterial occlusion (often noted in diabetes) is actually a large pale infarct. On gross examination, 1 or 2 days after the initial hyperemia, an infarct becomes soft, sharply delineated and light yellow (Fig. 7-20). The border tends to be dark red, reflecting hemorrhage into surrounding viable tissue. Microscopically, a pale infarct exhibits uniform coagulative necrosis. Red infarcts may result from either arterial or venous occlusion and are also characterized by coagulative necrosis.


A

279

B FIGURE 7-18. Talc emboli. A section of lung from an intravenous drug abuser shows talc particles before (A) and after (B) polarization of light.

However, they are distinguished by bleeding into the necrotic area from adjacent arteries and veins. Red infarcts occur principally in organs with a dual blood supply, such as the lung, or those with extensive collateral circulation, such as the small intestine and brain. In the heart, red infarcts occur when the infarcted area is reperfused, as may occur after spontaneous or therapeutically induced lysis of an occluding thrombus. Grossly, red infarcts are sharply circumscribed, firm and dark red to purple (Fig. 7-21). Over a period of several days, acute inflammatory cells infiltrate the necrotic area from the viable border. The cellular debris is phagocytosed and digested by polymorphonuclear leukocytes and later by macrophages. Granulation tissue eventually forms, to be replaced ultimately by a scar (see Chapter 3). In a large infarct of an organ such as the heart or kidney, the necrotic center may remain inaccessible to inflammatory cells and so persist for months. In the brain, an infarct typically undergoes liquefactive necrosis and may become a fluid-filled cyst, which is referred to as a cystic infarct (Fig. 7-22). Septic infarction results when the necrotic tissue of an infarct is seeded by pyogenic bacteria and becomes infected. Pulmonary infarcts are not uncommonly infected, presumably because the necrotic tissue offers little resistance to

FIGURE 7-19. Spleen infarcts. A cut section of spleen displays multiple pale, wedge-shaped infarcts beneath the capsule.

FIGURE 7-20. Acute myocardial infarct. A cross-section of the left ventricle reveals a sharply circumscribed, soft, yellow area of necrosis in the posterior free wall (arrows).

FIGURE 7-21. Red infarct. A sagittal slice of lung shows a hemorrhagic infarct in upper segments of the lower lobe.

7 | Hemodynamic Disorders

CHAPTER 7: HEMODYNAMIC DISORDERS | INFARCTION


280

RUBIN’S PATHOLOGY

FIGURE 7-22. Cystic infarct. A cross-section of brain in the frontal plane shows a healed cystic infarct.

inhaled bacteria. In the case of bacterial endocarditis, the emboli themselves are infected and resulting infarcts are often septic. A septic infarct may become an abscess (Fig. 7-23).

The Outcome of Infarction Depends on the Organ Involved and the Extent of Injury Myocardial Infarcts Myocardial infarcts are transmural (through the entire wall) or subendocardial. A transmural infarct results from complete occlusion of a major extramural coronary artery. Subendocardial infarction reflects prolonged ischemia caused by partially occluding, atherosclerotic, stenotic lesions of the coronary arteries when the requirement for oxygen exceeds the supply. Such a situation prevails in disorders such as shock, anoxia or severe tachycardia (rapid pulse). A myocar-

FIGURE 7-23. Septic infarct. A myocardial abscess (arrow) within the left ventricular free wall was due to infection with Staphylococcus aureus.

dial infarct may be pale or red, depending on the extent of reflow of blood into the infarcted area (Fig. 7-24).

Pulmonary Infarcts Only about 10% of pulmonary emboli elicit clinical symptoms referable to pulmonary infarction, usually after occlusion of a middle-sized pulmonary artery. Infarction occurs only if circulation from bronchial arteries is inadequate to compensate for supply lost from the pulmonary arteries. This occurs most often in congestive heart failure, although stasis in the pulmonary circulation may contribute. Hemorrhage into the alveolar spaces of the necrotic lining tissue occurs within 48 hours. FIGURE 7-24. Myocardial infarct. Transverse sections of ventricular myocardium show (A) reperfused, (B) acute (arrow) and healed (arrowhead) together and (C) healed infarct. Reperfusion is typically associated with hemorrhage as in A (arrow) and B (arrow). In C, a white scar (arrowhead) is evident in the anterior ventricular septum.

A

B

C


Cerebral Infarcts Infarction of the brain may result from local ischemia or a generalized reduction in blood flow. The latter often results from systemic hypotension, as in shock, and produces infarction in the border zones between the distributions of the major cerebral arteries (watershed infarct). If prolonged, severe hypotension can cause widespread brain necrosis. Occlusion of a single vessel in the brain (e.g., after an embolus has lodged) causes ischemia and necrosis in a well-defined area. This type of cerebral infarct may be pale or red, the latter being common with embolic occlusions. Occlusion of a large artery causes extensive necrosis, which may ultimately resolve as a large fluid-filled cavity in the brain.

Intestinal Infarcts The earliest tissue changes in intestinal ischemia are necrosis of the tips of the villi in the small intestine and of the superficial mucosa in the large intestine. More severe ischemia leads to hemorrhagic necrosis of the submucosa and muscularis, but not the serosa. Small mucosal infarcts heal within a few days, but more severe injury leads to ulceration. These ulcers can eventually reepithelialize. However, if ulcers are large, they are repaired by scarring, a process that may lead to strictures. Severe transmural necrosis is associated with massive bleeding or bowel perforation, complications that often result in irreversible shock, sepsis and death.

Edema Edema is excess fluid in interstitial tissue spaces. It may be local or generalized. Local edema in most instances occurs with inflammation, the “tumor” of “tumor, rubor and calor.” Local edema of a limb, usually the leg, results from venous or lymphatic obstruction. Burns cause prominent local edema by altering the permeability of local vasculature. Local edema may be a prominent component of an immune reaction, like urticaria (hives) or edema of the epiglottis or larynx (angioneurotic edema). Generalized edema, affecting visceral organs and the skin of the trunk and lower extremities (Fig. 7-25), usually reflects a global disorder of fluid and electrolyte metabolism, most often due to heart failure. Generalized edema is also seen when blood oncotic pressure is reduced, as in renal diseases in which serum proteins are lost in the urine (nephrotic syn-

281

drome; see Chapter 16) and in cirrhosis of the liver, when production of serum proteins may be impaired. Anasarca is extreme generalized edema, with conspicuous fluid accumulation in subcutaneous tissues, visceral organs and body cavities. Edema fluid may also collect in body cavities, such as the pleural space (hydrothorax), peritoneum (ascites) or pericardial sac (hydropericardium).

Normal Capillary Filtration Normal formation and retention of interstitial fluid depends on filtration and reabsorption at the level of the capillaries (Starling forces). Internal or hydrostatic pressure in the arteriolar segment of the capillary is 32 mm Hg. At the middle of the capillary, it is 20 mm Hg. Since interstitial hydrostatic pressure is only 3 mm Hg, the pressure differential causes outward fluid filtration at a rate of 14 mL/min. Hydrostatic pressure is opposed by plasma oncotic pressure (26 mm Hg), which results in osmotic reabsorption at 12 mL/min at the venous end of the capillary. Thus, interstitial fluid is formed at the rate of 2 mL/min and is reabsorbed by the lymphatics, so that in equilibrium there is no net fluid gain or loss in the interstitium.

Sodium and Water Metabolism Water represents 50% to 70% of body weight and is divided between extracellular and intracellular fluid spaces. Extracellular fluid is further divided into interstitial and vascular compartments. Interstitial fluid constitutes about 75% of the latter. Total body sodium is the principal determinant of extracellular fluid volume because it is the major cation in the extracellular fluid. In other words, increased total body sodium must be balanced by more extracellular water to maintain constant osmolality. Control of extracellular fluid volume depends to a large extent on regulation of renal sodium excretion, which is influenced by (1) atrial natriuretic factor, (2) the renin–angiotensin system of the juxtaglomerular apparatus and (3) sympathetic nervous system activity (see Chapter 10).

Edema Caused by Increased Hydrostatic Pressure Unopposed increases in hydrostatic pressure result in greater filtration of fluid into the interstitial space and its retention as edema. This happens in decompensated heart disease, in FIGURE 7-25. Pitting edema of the leg. A. In a patient with congestive heart failure, severe edema of the leg is demonstrated by applying pressure with a finger. B. The resulting “pitting” reflects the inelasticity of the fluid-filled tissue.

A

B

7 | Hemodynamic Disorders

CHAPTER 7: HEMODYNAMIC DISORDERS | EDEMA


282

RUBIN’S PATHOLOGY of plasma proteins, especially albumin. Any condition that decreases plasma albumin, whether it is albuminuria in the nephrotic syndrome or reduced albumin synthesis in chronic liver disease or severe malnutrition, promotes generalized edema.

Edema Caused by Lymphatic Obstruction Under normal circumstances, more fluid is filtered into the interstitial spaces than is reabsorbed into the vascular bed. This excess interstitial fluid is removed by lymphatics. Thus, obstruction to lymphatic flow leads to localized edema. Lymphatic channels can be obstructed by (1) malignant neoplasms, (2) fibrosis resulting from inflammation or irradiation and (3) surgical ablation. For instance, the inflammatory response to filarial worms (Bancroftian and Malayan filariasis; see Chapter 9) can result in lymphatic obstruction that produces massive lymphedema of the scrotum and legs (elephantiasis) (Fig. 7-26). Lymphedema of the arm often complicates radical mastectomies for breast cancer, due to removal of axillary lymph nodes and lymphatics. Lymphatic edema differs from other forms of edema in its high protein content, since lymph returns proteins and interstitial cells to the circulation. The high protein concentration of lymphedema may stimulate dermal fibrosis, which occurs in chronic edema (indurated edema).

The Role of Sodium Retention in Edema

FIGURE 7-26. Edema secondary to lymphatic obstruction. Massive edema of the right lower extremity (elephantiasis) in a patient with obstruction of lymphatic drainage.

which back-pressure in the lungs due to left ventricle failure leads to acute pulmonary edema and right-sided heart failure, and contributes to systemic edema. Similarly, back-pressure caused by venous obstruction in the lower extremity causes edema of the leg. Obstruction to portal blood flow in cirrhosis of the liver contributes to formation of abdominal fluid (ascites).

Edema Caused by Decreased Oncotic Pressure The difference in pressure between intravascular and interstitial compartments is largely determined by the concentration

Generalized edema and ascites invariably reflect increased total body sodium, as a consequence of renal sodium retention. By the time peripheral edema is first detectable clinically, the extracellular fluid volume has already expanded by at least 5 L. The most common conditions in which generalized edema is found include congestive heart failure, hepatic cirrhosis, nephrotic syndrome and some cases of chronic renal insufficiency. The mechanisms of edema formation and representative disorders associated with them are summarized in Fig. 7-27 and Table 7-1.

Congestive Heart Failure Is a Consequence of Inadequate Cardiac Output It is estimated that 2 to 3 million people in the United States have congestive heart failure. Of these, 15% die annually. Half of all patients with congestive heart failure who require admission to the hospital will die within 1 year. Congestive heart failure may be caused by any cardiac disease, but is most commonly associated with chronic cardiac ischemia (see Chapter 11).

FIGURE 7-27. The capillary system and mechanisms of edema formation. A. Normal. The differential between the hydrostatic and oncotic pressures at the arterial end of the capillary system is responsible for the filtration into the interstitial space of approximately 14 mL of fluid per minute. This fluid is reabsorbed at the venous end at the rate of 12 mL/min. It is also drained through the lymphatic capillaries at a rate of 2 mL/min. Proteins are removed by the lymphatics from the interstitial space. B. Hydrostatic edema. If the hydrostatic pressure at the venous end of the capillary system is elevated, reabsorption decreases. As long as the lymphatics can drain the surplus fluid, no edema results. If their capacity is exceeded, however, edema fluid accumulates. C. Oncotic edema. Edema fluid also accumulates if reabsorption is diminished by decreased oncotic pressure of the vascular bed, owing to a loss of albumin. D. Inflammatory and traumatic edema. Edema, either local or systemic, results if the vascular bed becomes leaky following injury to the endothelium. E. Lymphedema. Lymphatic obstruction causes the accumulation of interstitial fluid because of insufficient reabsorption and deficient removal of proteins, the latter increasing the oncotic pressure of the fluid in the interstitial space. !


283

7 | Hemodynamic Disorders

CHAPTER 7: HEMODYNAMIC DISORDERS | EDEMA


284

RUBIN’S PATHOLOGY

Table 7-1

Disorders Associated With Edema Increased hydrostatic pressure Arteriolar dilation

Inflammation Heat

Increased venous pressure

Venous thrombosis Congestive heart failure Cirrhosis (ascites) Postural inactivity (e.g., prolonged standing)

Hypervolemia

Sodium retention (e.g., decreased renal function)

Decreased oncotic pressure Hypoproteinemia

Increased capillary permeability

Lymphatic obstruction

Nephrotic syndrome Cirrhosis Protein-losing gastroenteropathy Malnutrition Inflammation Burns Adult respiratory distress syndrome Cancer Postsurgical lymphedema Inflammation

ETIOLOGIC FACTORS: The argument regarding the relative contributions of “forward failure” (low cardiac output) versus “backward failure” (venous congestion) in the pathogenesis of edema in congestive heart failure is no longer a burning issue. Both systolic and diastolic dysfunction contribute to the low cardiac output and high ventricular filling pressure characteristic of congestive heart failure, although systolic dysfunction is more important in most patients. Inadequate cardiac output in congestive heart failure leads to decreased glomerular filtration and increased renin secretion. The latter activates angiotensin, leading to release of aldosterone, subsequent sodium reabsorption and fluid retention. Furthermore, reduced hepatic blood flow impairs catabolism of aldosterone, further raising its concentration in the blood. Adequate intracardiac pressure is maintained by increased fluid volume, to compensate. In addition, increased sympathetic discharge leads to augmented levels of catecholamines, to stimulate cardiac contractility and further counteract the impairment in cardiac performance. Increased distention of the atria by the greater blood volume promotes release of atrial natriuretic peptide, which stimulates renal sodium excretion. With long-standing heart failure, these compensatory mechanisms fail. Then, renal sodium retention causes further expansion of plasma volume, which in turn leads to increased pulmonary and systemic venous pressure. Consequent increased hydrostatic pressure in the respective capillary beds, together with decreased plasma oncotic pressure, results in the edema of congestive heart failure.

PATHOLOGY: Left ventricular failure is associated principally with passive congestion of the lungs and pulmonary edema (Fig. 7-28). When chronic, these conditions lead to pulmonary hypertension and eventual failure of the right ventricle. Right ventricular failure is characterized by generalized subcutaneous edema (most prominent in the dependent portions of the body), ascites and pleural effusions. The liver, spleen and other splanchnic organs are typically congested. At autopsy, the heart is enlarged and its chambers dilated CLINICAL FEATURES: The effects of heart failure

depend on which ventricle is failing, recognizing that both may be failing simultaneously. Patients in leftsided heart failure complain of shortness of breath (dyspnea) on exertion or when recumbent (orthopnea). They may be awakened from sleep by sudden episodes of shortness of breath (paroxysmal nocturnal dyspnea). Physical examination usually reveals distended jugular veins. Persons with right-sided failure have pitting edema of the legs and an enlarged and tender liver. If ascites is present, the abdomen is distended. Patients in congestive heart failure with pulmonary edema have crackling breath sounds (rales) caused by expansion of fluid-filled alveoli.

Pulmonary Edema Features Increased Fluid in the Alveolar Spaces and Interstitium of the Lung This condition leads to decreased gas exchange in the lung, causing hypoxia and retention of carbon dioxide (hypercapnia).

FIGURE 7-28. Pathologic consequences of chronic congestive heart failure.


CHAPTER 7: HEMODYNAMIC DISORDERS | EDEMA

285

ETIOLOGIC FACTORS AND PATHOLOGY:

The lung is a loose tissue without much connective tissue support and, therefore, requires certain conditions to prevent the development of edema. Among these protective devices are:

Low perfusion pressure in lung capillaries, owing to low right ventricular pressure Effective drainage of the interstitial space of the lung by lymphatics, which are under a slightly negative pressure and can accommodate up to 10 times the regular lymph flow Tight cellular junctions between endothelial cells, which control capillary permeability

Pulmonary edema results if these protective mechanisms are disturbed. The most common causes of pulmonary edema relate to hemodynamic alterations in the heart that increase perfusion pressure in pulmonary capillaries and block effective lymphatic drainage. These conditions include left ventricular failure (the most common cause), mitral stenosis and mitral insufficiency. Disruption of capillary permeability is the cause of pulmonary edema in acute lung injury associated with adult respiratory distress syndrome, inhalation of toxic gases, aspiration of gastric contents, viral infections and uremia. Acute lung injury entails destruction of endothelial cells or disruption of their tight junctions (see Chapter 12). Pulmonary edema may be interstitial or alveolar. Interstitial edema is the earliest phase and is an exaggeration of normal fluid filtration. Lymphatics become distended and fluid accumulates in the interstitium of lobular septa and around veins and bronchovascular bundles. Radiologic examination reveals a reticulonodular pattern, more marked at lung bases. Lobular septa become edematous and produce linear shadows (“Kerley B lines”) on chest radiographs. Edema results in shunting of blood flow from the bases to the upper lobes. Edema of the bronchovascular tree increases resistance to airflow. Patients are often asymptomatic in this early stage. When the fluid can no longer be accommodated in the interstitial space, it spills into the alveoli, a condition termed alveolar edema. At this stage, a radiologic alveolar pattern is seen, usually worse in central and lower portions of the lung. The patient becomes acutely short of breath and bubbly rales are heard. In extreme cases, frothy fluid is coughed up or wells up out of the trachea. Microscopic examination of the edematous lung reveals severely congested alveolar capillaries and alveoli filled with a homogeneous, pink-staining fluid permeated by air bubbles (see Fig. 7-3). If pulmonary edema is caused by alveolar damage, cell debris, fibrin and proteins form films of proteinaceous material, called hyaline membranes, in the alveoli (Fig. 7-29). CLINICAL FEATURES: Pulmonary fluid accumu-

lation may go unnoticed at first, but eventually dyspnea and coughing become prominent. If edema is severe, large amounts of frothy pink sputum are expectorated. Hypoxemia is manifested as cyanosis. Pulmonary function is restricted in severe congestion and in interstitial pulmonary edema because fluid accumulation in the interstitial space reduces pulmonary compliance (i.e., it makes the lung stiffer). Thus, increased respiratory work is required to maintain ventilation. Thickened alveolar walls

7 | Hemodynamic Disorders

FIGURE 7-29. Pulmonary edema due to diffuse alveolar damage. A section of lung shows hyaline membranes (arrows) in alveoli.

represent a greater barrier to oxygen and carbon dioxide exchange. The latter is less affected than the former, resulting in hypoxia with near-normal carbon dioxide levels. Mismatch between ventilation (which is reduced) and perfusion (which persists) leads to hypoxemia in patients with pulmonary edema.

Edema in Cirrhosis of the Liver Is Commonly an End-Stage Condition Cirrhosis of the liver is often accompanied by ascites and peripheral edema (see Chapter 14). Liver scarring obstructs portal blood flow and leads to portal hypertension and increased hydrostatic pressure in the splanchnic circulation. This situation is compounded by decreased hepatic synthesis of albumin as a result of liver dysfunction. Consequent accumulation of peritoneal fluid decreases effective blood volume, leading to renal sodium retention by mechanisms similar to those operative in congestive heart failure. Alternatively, chronic liver disease itself causes renal sodium retention. Subsequent expansion of extracellular fluid volume further promotes ascites and edema, thus establishing a vicious circle. In addition, increased transudation of lymph from the liver capsule adds to accumulation of fluid in the abdomen.

In Nephrotic Syndrome, the Rate of Protein Loss in the Urine Exceeds the Rate of Protein Replacement by the Liver The resulting decline in the concentration of plasma proteins, particularly albumin, reduces plasma oncotic pressure and


286

RUBIN’S PATHOLOGY

promotes edema. Blood volume therefore decreases, stimulating the renin–angiotensin–aldosterone mechanism and leading to sodium retention. The edema is generalized but appears preferentially in soft connective tissues, the eyes, eyelids and subcutaneous tissues. Ascites and pleural effusions also occur.

Cerebral Edema Often Causes a Fatal Increase in Intracranial Pressure Edema of the brain is dangerous because the rigidity of the cranium allows little room for the brain to expand. Increased intracranial pressure from edema compromises cerebral blood supply, distorts the gross structure of the brain and interferes with central nervous system function (see Chapter 28). Cerebral edema is divided into vasogenic, cytotoxic and interstitial forms. ■

Vasogenic edema, the most common variety of edema, is excess fluid in the extracellular space of the brain. It results from increased vascular permeability, mainly in white matter. The tight endothelial junctions of the bloodbrain barrier are disrupted and fluid filters into the interstitial space. Disorders causing cerebral vasogenic edema include trauma, neoplasms, encephalitis, abscesses, infarcts, hemorrhage and toxic brain injury (e.g., lead poisoning). Cytotoxic edema is equivalent to hydropic cell swelling (i.e., accumulation of intracellular water). It is usually a response to cell injury, such as that produced by ischemia. Cytotoxic cerebral edema preferentially affects the gray matter. Interstitial edema is a consequence of hydrocephalus, in which fluid accumulates in the cerebral ventricles and periventricular white matter.

At autopsy, an edematous brain is soft and heavy. Gyri are flattened and sulci narrowed. Because of alterations in brain function, patients with cerebral edema suffer vomiting, disorientation and convulsions. Severe cerebral edema leads to herniation of the cerebellar tonsils, ordinarily a lethal event.

Fluid Accumulates in Body Cavities as Extensions of the Interstitial Space The Pleural Space Pleural effusion (fluid in the pleural space) is a straw-colored transudate of low specific gravity that contains few cells (mainly exfoliated mesothelial cells). Fluid commonly accumulates as an expression of a generalized tendency to form edema in diseases such as the nephrotic syndrome, cirrhosis of the liver and congestive heart failure. Pleural effusions often accompany inflammatory processes or tumors in the lung or on the pleural surface.

The Pericardium Fluid in the pericardial sac may result from hemorrhage (hemopericardium) or injury to the pericardium (pericardial effusion). Pericardial effusions occur with pericardial infections, tumors metastatic to the pericardium, uremia and inflammatory conditions such as systemic lupus erythematosus. Pericardial fluid accumulation may also occur after

FIGURE 7-30. Cardiac tamponade. A cross-section of the heart shows rupture of a myocardial infarct (arrow) with the accumulation of a large quantity of blood in the pericardial cavity.

cardiac operations (postpericardiotomy syndrome) or radiation therapy for cancer. If pericardial fluid accumulates rapidly (e.g., with hemorrhage from a ruptured myocardial infarct, dissecting aortic aneurysm or trauma), the pressure in the pericardial cavity may exceed the filling pressure of the heart. This condition, termed cardiac tamponade (Fig. 7-30), leads to a precipitous decline in cardiac output and is often fatal. If pericardial fluid accumulates rapidly, the tolerable limit may be only 90 to 120 mL, but a liter or more of fluid can be accommodated if the process is gradual.

Peritoneum Peritoneal effusion, also called ascites, is caused mainly by hepatic cirrhosis, abdominal tumors, pancreatitis, cardiac failure, the nephrotic syndrome and hepatic venous obstruction (Budd-Chiari syndrome). Obstruction of the thoracic duct by cancer may lead to chylous ascites, in which the fluid has a milky appearance and a high fat content. The pathogenesis of ascites in cirrhosis of the liver is discussed above. Patients with severe ascites accumulate many liters of fluid and have hugely distended abdomens. Complications of ascites reflect increased abdominal pressure and include anorexia and vomiting, reflux esophagitis, dyspnea, ventral hernia and leakage of fluid into the pleural space.

Fluid Loss and Overload Fluid imbalance, whether excessive loss (dehydration) or overload, has potentially grave consequences. It causes hemodynamic disorders; alterations in osmolality and the quantity of fluid in intravascular, interstitial and cellular spaces may affect perfusion or delivery of nutrients, electrolytes or fluids.

Dehydration Results From Insufficient Fluid Intake, Excessive Fluid Loss or Both Water loss may exceed intake in cases of vomiting, diarrhea, burns, excessive sweating and diabetes insipidus. When excessive fluid loss occurs, fluid is recruited from the


CHAPTER 7: HEMODYNAMIC DISORDERS | SHOCK

CLINICAL FEATURES: Clinically, only dryness of

the skin and mucous membranes is noted initially, but as dehydration progresses, skin turgor is lost. If dehydration persists, oliguria (reduced urine output) occurs as a compensation for the fluid loss. More severe fluid loss is accompanied by a shift of water from the intracellular space to the extracellular space, leading to severe cell dysfunction, particularly in the brain. Shrinkage of brain tissue may cause rupture of small vessels and subsequent bleeding. Systemic blood pressure falls with continuous dehydration; declining perfusion eventually leads to death.

In Overhydration, Fluid Intake Exceeds Renal Excretory Capacity Overhydration is rare, unless injury to the kidneys limits their excretory function or they are prevented from proper counterregulation (e.g., via excessive secretion of antidiuretic hormone). Fluid overload today is mostly caused by administration of excessive amounts of intravenous fluids. The most serious effect of such fluid overload is induction of cerebral edema or congestive heart failure in patients with cardiac dysfunction.

MOLECULAR PATHOGENESIS:

Blood Pressure Control Twin and family studies suggest that genetics accounts for roughly 30% of blood pressure regulation (see Chapter 10). This may also account for the considerable variation in patient response to antihypertensive medication. Human genetic linkage and whole genome association studies have identified a host of mutations in key blood pressure regulatory processes. Prominent are genes of the renin–angiotensin system, which regulates vasoconstriction and sodium and water balance. Single nucleotide polymorphisms (SNPs) in genes encoding angiotensin, angiotensin-converting enzyme, angiotensin II receptor, renin and renin-binding protein are associated with altered blood pressure control. Hypertension has been associated with SNPs in the vasoconstrictor endothelin and its receptor, the vasodilator nitric oxide synthase and endothelial sodium channel subunits. Polymorphisms of !-adrenergic receptors 1 and 2 are associated with hypertension and altered response to !-agonists. Elucidation of the genetic bases for blood pressure regulation will improve our understanding of how blood pressure is controlled, identify patients at increased risk for development of hypertension and facilitate development of antihypertensive therapies.

Shock Shock is a profound hemodynamic and metabolic disturbance characterized by failure to maintain an adequate blood supply to the microcirculation, with consequent inadequate perfusion of vital organs. In this often catastrophic circumstance, tissue perfusion and oxygen delivery fall below levels required to meet normal demands, including failure to remove metabolites adequately. The term shock encompasses all the reactions that occur in response to such disturbances. During uncompensated shock, rapid circulatory collapse leads to impaired cellular metabolism and death. However, in many cases, compensatory mechanisms sustain the patient, at least for a while. When these adaptations fail, shock becomes irreversible. Shock has been a major cause of morbidity and mortality in intensive care units. Unfortunately, the outcome of shock has not changed appreciably in the past 50 years. Shock is not synonymous with low blood pressure, although hypotension is often part of the shock syndrome. Hypotension is actually a late sign in shock and indicates failure of compensation. At the same time that peripheral blood flow falls below critical levels, extreme vasoconstriction can maintain arterial blood pressure. The distinction between shock and hypotension is important clinically because rapid restoration of systemic blood flow is the primary goal in treating shock. If blood pressure alone is raised with vasopressive drugs, systemic blood flow may actually be diminished. ETIOLOGIC FACTORS: Decreased perfusion in shock most commonly results from decreased cardiac output, due either to the inability of the heart to pump normal venous return or to decreased effective blood volume that leads to decreased venous return. These two mechanisms underlie two of the major types of shock: cardiogenic and hypovolemic shock. Systemic vasodilation, with or without increased vascular permeability, is responsible for the other categories of shock: septic, anaphylactic and neurogenic shock (Fig. 7-31). ■

Cardiogenic shock is caused by myocardial pump failure. It usually arises after massive myocardial infarction, but myocarditis may also be responsible. Conditions that prevent left or right heart filling reduce cardiac output, resulting in “obstructive” shock. Such conditions include pulmonary embolism, cardiac tamponade (Fig. 7-30) and (rarely) atrial myxoma. Hypovolemic shock is secondary to a pronounced decrease in blood or plasma volume, caused by loss of fluid from the vascular compartment. Hemorrhage, fluid loss from severe burns, diarrhea, excessive urine formation, perspiration and trauma are the major causes of fluid loss that can lead to hypovolemic shock. In the case of burns or trauma, direct damage to the microcirculation increases vascular permeability. Septic shock is caused by severe systemic microbial infections. The pathogenesis of septic shock is complex and is discussed in detail below. Anaphylactic shock is a consequence of a systemic type I hypersensitivity reaction, which leads to widespread vasodilation and increased vascular permeability. Neurogenic shock can follow acute injury to the brain or spinal cord, which impairs the neural control of vasomotor tone, causing generalized vasodilation. In the case of both anaphylactic and neurogenic shock, the subsequent

7 | Hemodynamic Disorders

interstitial space to the plasma space. Fluids in cells and within the interstitial and vascular compartments become more concentrated, particularly if there is a preferential loss of water, such as during inappropriate secretion of antidiuretic hormone in diabetes insipidus. Patients suffering from burns, vomiting, excessive sweating or diarrhea both lose fluid and have electrolyte disturbances. As a result, there is insufficient fluid to fill the fluid compartments of the body.

287


288

RUBIN’S PATHOLOGY

SHOCK

FIGURE 7-31. Classification of shock. Shock results from (1) an inability of the heart to pump adequately (cardiogenic shock), (2) decreased effective blood volume as a consequence of severely reduced blood or plasma volume (hypovolemic shock) or (3) widespread vasodilation (septic, anaphylactic or neurogenic shock). Increased vascular permeability may complicate vasodilation by contributing to reduced effective blood volume.

Cardiogenic

Hypovolemic

Septic

Anaphylactic

Neurogenic

Myocardial infarction

Hemorrhage

Severe infection

Type 1 hypersensitivit y reaction

Brain damage

Myocarditis Cardiac tamponade

Diarrhea Dehydration Burns

Pulmonary embolus

Myocardial Pump Failure

redistribution of blood to the periphery, with or without increased vascular permeability, reduces the effective circulating blood and plasma volume. This ultimately leads to the same consequences as in hypovolemic shock. In hypovolemic and cardiogenic shock, lower cardiac output and resultant decreased tissue perfusion are the key steps in the progression from reversible to irreversible shock. Cellular hypoxia is the common consequence of the initial decrease in tissue perfusion. Although such changes do not initially result in irreversible injury, a vicious circle of decreasing tissue perfusion and further cell injury is perpetuated by several mechanisms: ■

Spinal cord injury

Injury to endothelial cells, secondary to the hypoxia caused by decreased tissue perfusion and increased vascular permeability, leads to escape of fluid from the vascular compartment. Increased exudation of fluid from the circulation reduces (1) blood volume; (2) venous return; and (3) cardiac output, thereby aggravating hypoxic cell injury. Decreased perfusion of kidneys and skeletal muscles results in metabolic acidosis, which in turn further decreases cardiac output and tissue perfusion. Decreased perfusion of the heart injures myocardial cells and decreases their ability to pump blood, further reducing cardiac output and tissue perfusion.

Systemic Inflammatory Response Syndrome Characterizes Septic Shock Systemic inflammatory response syndrome (SIRS) is an exaggerated, generalized manifestation of a local immune or inflammatory reaction, and is often fatal. SIRS is a hypermetabolic state characterized by two or more signs of systemic inflammation—such as fever, tachycardia, tachypnea, leukocytosis or leukopenia—in the setting of a known cause of inflammation. Septic shock is defined as clinical SIRS so severe that it leads to organ dysfunction and hypotension. The mechanisms responsible for development of septic shock are illustrated in Fig. 7-32. These processes often progress to multiple organ dysfunction syndrome (MODS), a term used

Blood Volume

Vasodilation

Vascular Permeability

to describe otherwise unexplained abnormalities of organ function in critically ill patients (see below).

MOLECULAR PATHOGENESIS: The massive inflammatory reaction defined by SIRS results from systemic release of cytokines, the most important being tumor necrosis factor (TNF), interleukin1 (IL-1), IL-6 and platelet-activating factor (PAF). Over 30 endogenous mediators of SIRS have been identified. Their interactions may be important in the pathogenesis of SIRS. Septicemia with gram-negative organisms is the most common cause of septic shock. The invading bacteria release endotoxin, a lipopolysaccharide (LPS), whose toxic activity resides in the lipid A component. On entry into the circulation, LPS, via lipid A, binds to LPS-binding protein. This complex binds to CD14 on the surface of monocyte/macrophages, which is part of a recognition complex that also includes the toll-like receptor (TLR) family of proteins and the recently discovered peptidoglycan recognition proteins (PGRP). TLRs are the primary sensors of the innate immune system, which collectively recognize bacteria, fungi and protozoa. Immediately downstream of TLR binding are the myeloid differentiation protein 88 (MyD 88), toll-interleukin-1 (TIR) domain-containing adaptor protein, TIR receptor domain-containing adaptor protein inducing interferon ! (TRIF) and TRIF-related adaptor molecule. These mediate signaling through activation of nuclear factor-"B (NF-"B) transcription factor and upregulate TNF expression. LPS binding to TLR-4 causes mononuclear phagocytes to secrete large quantities of cytokines, such as TNF, IL-1, IL-6, IL-8, IL-12, macrophage inhibitory factor and others, that mediate a variety of responses. These cytokines, and subsequent production of nitric oxide (NO•) and procoagulant proteins, ultimately cause the overwhelming cardiovascular collapse characteristic of septic shock. In this context activation of inducible NO synthase (iNOS) by TNF upregulates NO• synthesis from L-arginine, an effect that is primarily responsible for the drop in blood pressure that occurs


CHAPTER 7: HEMODYNAMIC DISORDERS | SHOCK

Recognition of Genetic Polymorphisms in TollLike Receptors and Tumor Necrosis Factor Has Helped Elucidate the Pathogenesis of Sepsis Gene mutations in several cytokines, cell surface receptors and other circulating markers have been associated with variability in susceptibility to sepsis. TLR-pattern recognition receptors recognize pathogen-associated microbial patterns and thus are critical in triggering innate immune responses. Toll-like receptor-4 (TLR4) is critical in recognizing LPS of gram-negative bacteria. A mutation, from aspartic acid to glycine at amino acid 299 of TLR4, leads to reduced inflammatory responses in a variety of clinical settings. Thus, TLR4 appears to be important in magnifying responses to endotoxin and in sepsis. Polymorphisms in TLRs and other pattern recognition receptors may help to explain why patients respond so differently to specific infectious agents. Similarly, recently discovered mutations in the TNF-! gene have improved our understanding of the role of TNF! in sepsis. For example, a G to A base change at base 308 of the TNF-! promoter leads to enhanced promoter activity

and increased expression of TNF-! and is associated with increased risk of sepsis and shock. Other gene mutations associated with worse prognosis in sepsis are found in IL-1 receptor agonist, CD14 and plasminogen activator inhibitor-1.

Multiple Organ Dysfunction Syndrome Is the End-Result of Shock Improvements in the early treatment of shock and sepsis have allowed patients to survive long enough to manifest a new problem, progressive deterioration of organ function. Almost all septic shock patients suffer from dysfunction of at least one organ. However, multiple organ dysfunction occurs in one third of patients with septic shock, trauma or burns and in a quarter of those with acute pancreatitis. Whatever the cause, the clinical deterioration of MODS is held to result from common mechanisms of tissue injury subsumed under the rubric of SIRS. Mortality of SIRS/MODS exceeds 50%, making it responsible for most deaths in noncoronary intensive care units in the United States. In most cases inflammatory reactions and the progression from sepsis to organ dysfunction reflect a balance between proinflammatory and anti-inflammatory factors. As mentioned above, TNF-!, IL-1 and NO• have systemic effects. Also, reactive oxygen species are important triggers of end-organ dysfunction. The acute response to sepsis is characterized by release of adrenocorticotropic hormone, cortisol, adrenaline and noradrenaline, vasopressin, glucagon and growth hormone. The net result is shutdown of noncritical systems and an overall catabolic state. Although proinflammatory mediators predominate in SIRS, anti-inflammatory factors play an important role in some patients. The result is compensated anti-inflammatory response syndrome (CARS), in which paralysis of the immune system leads to a poor outcome. It is now thought that following bacterial infection, there is an initial response of excessive inflammation and septic shock characteristic of SIRS. Such uncontrolled cytokine induction is preceded by a stage of anergy and immune repression or CARS. Septic patients who cycle between SIRS and CARS are susceptible to increased mortality. Persons with a heterogeneous response are said to have a “mixed anti-inflammatory response syndrome” (MARS).

Vascular Compensatory Mechanisms Changes in the macrovascular and microvascular circulation are at least partly responsible for variable organ injury in SIRS. Compensatory mechanisms in shock shift blood flow away from the periphery, so as to maintain flow to the heart and the brain. These responses involve the sympathetic nervous system, release of endogenous vasoconstrictors and hormonal substances, and local vasoregulation. The result is increased cardiac output achieved by increasing heart rate and myocardial contractility while constricting arteries and arterioles. ■

Increased sympathetic discharge augments catecholamine release by the adrenal medulla. Skeletal muscle, splanchnic bed and skin arterioles respond to increased sympathetic discharge; cardiac and cerebral arterioles are less reactive. Thus, increased sympathetic tone shifts blood flow from the periphery to the heart and brain. The marked arteriolar

7 | Hemodynamic Disorders

during sepsis. TNF is also central to the pathogenesis of shock that is not associated with endotoxemia (e.g., cardiogenic shock). While LPS is the most potent stimulus, other antigens also promote TNF release. These include toxin-1 of the toxic shock syndrome; enterotoxin; antigens of mycobacteria, fungi, parasites and viruses; and products of complement activation. When macrophages are exposed to LPS in septic shock, large amounts of TNF are suddenly released, often with lethal consequences. Administering anti-TNF antibody before exposing an animal to endotoxin or to gramnegative bacteria completely protects from septic shock. Unfortunately, comparable studies in humans have not been as successful. TNF released by monocyte/macrophages exerts a direct toxic effect on endothelial cells by compromising membrane permeability and inducing endothelial cell apoptosis. It also acts indirectly by (1) initiating a cascade of other mediators that amplify its deleterious effects, (2) promoting adhesion of polymorphonuclear leukocytes to endothelial surfaces and (3) activating the extrinsic coagulation pathway. TNF stimulates release of IL-1 and IL-6, PAF and other eicosanoids that may mediate tissue injury. Interestingly, in animal studies, nonlethal doses of TNF may become fatal if administered together with IL-1. TNF also increases expression of adhesion molecules, such as intercellular adhesion molecules (ICAMs), vascular cell adhesion molecules (VCAMs), P-selectin and endothelial-leukocyte adhesion molecules (ELAMs) on endothelial surfaces, thereby promoting leukocyte adhesion and leukostasis. This mechanism presumably plays a role in the respiratory distress syndrome, in which activated neutrophils are sequestered in the pulmonary circulation and damage alveoli. Other vasoactive peptides include the vasodilatory prostacyclins and endothelin (ET)-1, a potent vasoconstrictor (Fig. 7-32). Note that the term septic syndrome refers to the physiologic and metabolic response characteristic of sepsis in the absence of an infection.

289


290

RUBIN’S PATHOLOGY

Gram negative and positive bacteria and other organisms

Bacterial products

Anti-inflammatory mediators

IL-10 Pattern recognition receptor Pro-inflammatory mediators: IL-6 IL-8

Monocyte/ macrophage Polymorphonuclear leukocyte

TNF-α

β2 -integrins CD11/CD18

Tissue factor Reactive oxygen intermediates

Endothelial pro-coagulation iNOS Endothelial cells NO•

Vasodilatation

Adhesion molecules; P-selectin, ICAM-1, VCAM-1

Endothelial activation

Ca2+ Smooth muscle cells

Parenchymal cells

Chemokines: IL-8, MIP-2, MCP-1

Chemotaxis of leukocytes toward locale for resolution and repair

FIGURE 7-32. Pathogenesis of endotoxic shock. Sepsis is caused primarily by gram-negative bacteria and bacterial products such as endotoxin (lipopolysaccharide [LPS]), which is released into the circulation, where it binds to a pattern recognition receptor on the surface of monocyte/macrophages. Such binding stimulates the secretion of substantial quantities of tumor necrosis factor-! (TNF-!). TNF-! mediates septic shock by a number of mechanisms: (1) stimulation of the release of various pro- and anti-inflammatory mediators; (2) induction of endothelial procoagulation by tissue factor, thereby leading to thrombosis and local ischemia; (3) direct cytotoxic damage to endothelial cells; (4) endothelial activation, which enhances the adherence of polymorphonuclear leukocytes; (5) stimulation of endothelial cell nitric oxide production and vasodilation; and (6) release of chemokines to attract leukocytes for resolution and repair of tissue injury. Ca2" # calcium ion; ICAM # intercellular adhesion molecule; IL # interleukin; iNOS # inducible nitric oxide synthetase; MCP-1 # monocyte chemotactic protein-1; MIP-2 # macrophage-inflammatory protein-2; NO• # nitric oxide; VCAM-1 # vascular cell adhesion molecule-1.


CHAPTER 7: HEMODYNAMIC DISORDERS | SHOCK

PATHOLOGY: Shock is associated with specific changes in a number of organs (Fig. 7-33), including acute renal tubular necrosis, acute respiratory distress syndrome, liver failure, depression of host defense mechanisms and heart failure. Interestingly, paracrine crosstalk from one injured organ such as proinflammatory mediators from the lung can affect distant organ injury.

Heart Dysfunction in both systolic and diastolic circuits is associated with sepsis and most likely reflects paracrine injury and possibly hypoperfusion. In sepsis, the heart shows petechial hemorrhages of the epicardium and endocardium. Microscopically, necrotic foci in the myocardium range from loss of single fibers to large areas of necrosis. Prominent contraction

bands are visible by light microscopy but are better seen by electron microscopy. Ultrastructurally, flattened areas of the intercalated disk are a sign of cell swelling, and invagination of adjacent cells is considered to be a catecholamine-induced lesion.

Kidney Acute tubular necrosis (acute renal failure), a major complication of shock, has been divided into three phases: (1) initiation, from the onset of injury to the beginning of renal failure; (2) maintenance, from the onset of renal failure to a stable, reduced renal function; and (3) recovery. In those who survive an episode of shock, the recovery phase begins about 10 days after its onset and may last up to 8 weeks. Renal blood flow is restricted to one third of normal following the acute ischemic phase. This effect is even more severe in the outer cortex. Constriction of arterioles reduces filtration pressure, thereby reducing the volume of filtrate and contributing to oliguria. Interstitial edema occurs, possibly through a process termed backflow. Excessive vasoconstriction is also related to stimulation of the renin–angiotensin system. During acute renal failure, the kidney is large, swollen and congested, although the cortex may be pale. Cross-section reveals blood pooling in the outer stripe of the medulla. Microscopically, fully developed acute tubular necrosis entails dilation of the proximal tubules and focal necrosis of cells (Fig. 7-34). Frequently, pigmented casts in tubular lumina indicate leakage of hemoglobin or myoglobin. Coarse, “ropy” casts are seen in the distal nephron and distal convoluted tubules. Interstitial edema is prominent in the cortex and mononuclear cells accumulate within tubules and surrounding interstitium. Acute tubular necrosis is discussed in more detail in Chapter 16.

Lung After the onset of severe and prolonged shock, injury to alveolar walls can result in shock lung, which is a cause of acute

Fever, brain death

Adult respiratory distress syndrome (ARDS) Centrilobular hemorrhagic necrosis of liver

Focal myocardial necrosis Congestion and hyperplasia of spleen

Acute tubular necrosis of kidney Superficial hemorrhagic necrosis of intestine

Stress (steroid) ulcers of stomach Vasodilatation and spanchnic pooling

FIGURE 7-33. Complications of shock.

FIGURE 7-34. Acute tubular necrosis. A section of kidney shows swelling and degeneration of tubular epithelium. Arrows indicate the thinned and damaged epithelium.

7 | Hemodynamic Disorders

vasoconstriction reduces capillary hydrostatic pressure and decreases fluid shifted into the interstitium, facilitating an osmotic fluid shift from the interstitium to the vascular system. This sympathetic–adrenal response can compensate completely for loss of 10% of intravascular blood volume. If more fluid is lost, cardiac output and blood pressure are affected and blood flow to tissues is reduced. The renin–angiotensin–aldosterone system also helps compensate, by stimulating sodium and water reabsorption, thereby helping to maintain intravascular volume. A similar water-preserving action is provided by pituitary antidiuretic hormone. Vascular autoregulation preserves regional blood flow to vital organs, particularly the heart and brain, by vasodilation in the coronary and cerebral circulations in response to hypoxia and acidosis. Vasoconstriction mediated largely by !-adrenergic receptors in mesenteric venules and veins helps maintain cardiac filling and arterial pressure. Circulation to organs such as skin and skeletal muscles, which are less sensitive to hypoxia, does not display such tightly controlled autoregulation.

291


292

RUBIN’S PATHOLOGY

respiratory distress syndrome (ARDS) (see Chapter 12). The sequence of changes is mediated by polymorphonuclear leukocytes and includes interstitial edema, necrosis of endothelial and alveolar epithelial cells and formation of intravascular microthrombi and hyaline membranes lining the alveolar surface. Macroscopically, the lung is firm and congested and a frothy fluid often exudes from the cut surface. Interstitial edema is first seen around peribronchial connective tissue and lymphatics, subsequently filling the interstitial connective tissue. In this initial period, a large fluid volume drains into the pulmonary lymphatics. Alveolar edema may develop if this fluid is not adequately removed or if the balance of forces that keep the fluid in the interstitial space is disturbed. Shock-induced lung injury leads to so-called alveolar hyaline membranes (see Fig. 7-29), which also frequently line alveolar ducts and terminal bronchioles. These changes may heal entirely, but in half of patients, repair processes cause thickening of the alveolar wall. Type II pneumocytes proliferate to replace damaged type I pneumocytes and line the alveoli. Fibrous tissue proliferation may lead to organization of the alveolar exudate. These chronic changes may result in persistent respiratory distress and even death. Shock lung and ARDS are more fully discussed in Chapter 12.

Gastrointestinal Tract Shock often results in diffuse gastrointestinal hemorrhage. Erosions of the gastric mucosa and superficial ischemic necrosis in the intestines are the usual sources of this bleeding. Interruption of the barrier function of the intestine may lead to septicemia. More severe necrotizing lesions contribute to deterioration in the final phase of shock.

Liver In patients who die in shock, the liver is enlarged and has a mottled cut surface that reflects marked centrilobular pooling of blood. The most prominent histologic lesion is centrilobular congestion and necrosis. The basis for the apparent increased sensitivity of centrilobular hepatocytes to shock may not simply represent their greater distance from the source of blood delivered via the portal tracts, but may reflect variable metabolic susceptibilities among the different zones of the hepatic lobule, a matter that is not settled (see Chapter 14).

FIGURE 7-35. Waterhouse-Friderichsen syndrome. A normal adrenal gland (left) in contrast to an adrenal gland enlarged by extensive hemorrhage (right), obtained from a patient who died of meningococcemic shock.

Pancreas The splanchnic vascular bed, which supplies the pancreas, is particularly affected by impaired circulation during shock. Resulting ischemic damage to the exocrine pancreas unleashes activated catalytic enzymes and causes acute pancreatitis, which further promotes shock.

Brain Although septic patients often have clinical encephalopathy, discrete brain lesions are rare in SIRS and shock. Microscopic hemorrhages may be seen, but patients who recover do not ordinarily have neurologic deficits. In severe cases, particularly in persons with cerebral atherosclerosis, hemorrhage and necrosis may appear in the overlapping region between the terminal distributions of major arteries, so-called watershed infarcts (see Chapter 28).

Adrenals In severe shock, adrenal glands exhibit conspicuous hemorrhage in the inner cortex. The hemorrhage is often focal. However, it can be massive and accompanied by hemorrhagic necrosis of the entire gland, as seen in Waterhouse-Friderichsen syndrome (Fig. 7-35), typically associated with overwhelming meningococcal septicemia.

病理w3  
病理w3  
Advertisement